1. Field of the Invention
The invention relates generally to photographic cameras and in particular to those cameras which have a frame counter for indicating the number of frames exposed or remaining to be exposed on a filmstrip in the camera.
2. Description of the Prior Art
Typically in many recent 35 mm cameras, there is included a motorized film transport mechanism. In operation, a leading end portion of the filmstrip extending from a light-tight cartridge loaded in the camera is attached automatically to a take-up spool. The take-up spool is rotated automatically after each exposure to advance successive frames of the filmstrip from the cartridge across the focal plane of a taking lens and onto the take-up spool. Each time the take-up spool is rotated, an unexposed frame on the filmstrip is located in the focal plane of the taking lens and the exposed frame is wound onto the take-up spool. When the fresh film supply has been exhausted, a trailing end portion of the filmstrip remains attached to a dispensing spool within the cartridge. The end of film condition is signalled by a sudden rise in the film tension as the take-up spool attempts to withdraw the remainder of the filmstrip from the cartridge. A tension sensing mechanism responds to this increase in film tension by reversing the motor drive to rotate the dispensing spool. The rotated spool draws the filmstrip off the take-up spool and rewinds it into the cartridge. Then, a rear door of the camera is opened and the cartridge is removed in order to process the filmstrip.
Opening the rear door of the camera to remove the cartridge initializes a frame counter to an empty setting. When a cartridge with a fresh filmstrip is loaded in the camera, the counter is incremented from its empty setting to a "1" setting in response to advance of the filmstrip to locate its first frame in the focal plane of the taking lens. Failure to initialize the counter would result in the frame count being out of synchronization with the exposed frames on the filmstrip.
If the rear door of the camera is accidentally or inadvertently opened before the filmstrip is completely exposed or before it is substantially rewound back into the cartridge, the exposed frames on at least the outermost convolution of the filmstrip on the take-up spool will be ruined. To prevent this occurance, cameras have been recently provided with a motorized transport mechanism which pre-winds the filmstrip onto the take-up spool before any exposures are taken. Then, after each exposure the filmstrip is rewound incrementally into the cartridge. Thus the exposed frames will be protected within the cartridge should the rear door be opened prematurely.
A frame counter in a camera that pre-winds a fresh filmstrip onto the take-up spool and rewinds the filmstrip incrementally into the cartridge after each exposure is driven by film movement in both the pre-wind and rewind directions. When the filmstrip is moved in the pre-wind direction, the counter is incremented to indicate the maximum number of available exposures. Conversely, movement of the filmstrip in the rewind direction decrements the counter after each exposure to indicate the remaining number of available exposures. Opening the rear door of the camera accidentally or inadvertently before the filmstrip is completely wound off the take-up spool will initialize the counter to its empty setting. This may be considered undesireable because the counter cannot continue its frame count after the door is re-closed should the photographer wish to salvage any unexposed portion of the filmstrip beneath the outermost convolution on the take-up spool.
It has been suggested in commonly assigned U.S. Pat. No. 3,511,153, granted May 12, 1970, that a frame counter not be initialized to its empty setting in the event the rear door of the camera is opened before the filmstrip is completely rewound into the cartridge. Specifically, rotation of the counter to increment or decrement the counter is prevented by arresting a metering sprocket in a predetermined angular position. The metering sprocket is connected to the counter and has teeth engagable with successive perforations in the filmstrip to rotate the sprocket and the counter during film movement. However, with this arrangement, the counter cannot be initialized except by rotating the counter manually to its empty setting.